Green Screen

Blue or Green – what to choose?

This month we’ll be covering the process of creating green screen effects in detail, in honor of St Patrick’s day. Leading up to that, I thought I’d post a few thoughts on the logic behind the choice of bluescreen or greenscreen.

If you’ve been around the biz for a few years, you may have noticed a shift from using bluescreens to predominantly using green screens for VFX work. You may have even heard some reasons why. Those reasons are probably wrong.

Myth: Green is a better color for digital keying

One oft repeated rationale for the choice of green over blue is that green is stored at higher resolution in modern digital formats and so it’s the better choice for a key screen color. Like all good myths there’s an element of truth here.

image1-18Now hang in there for just a little geek-speak: it is true that in a common color space like 4:2:2 luminance is sampled at each pixel, while sub-carrier components are stored less frequently (for 4:2:2 that’s every two pixels). It’s also true that 59% of that luminance is composed of green light. But that doesn’t translate to a better resolution key when using a greenscreen over a bluescreen (or even a redscreen for that matter).

Matte artifacts resulting from a 4:1:1 subsampling

You see, in post-production a key is pulled by subtracting colors–if it’s a green screen, you’re subtracting the red and blue channels away from the green to get your key. Subtracting a low resolution color channel from a high resolution color channel will still give you a low resolution result. So whether you’re subtracting red and blue from green  (greenscreen) or subtracting red and green from blue (bluescreen), if some of the color is sampled at a lower resolution, the key will suffer.

So is green better?

OK, so is there any real reason to go with green over blue? Well, there are in fact a few reasons. The first is that green lighting typically has a much higher luminance level than blue. So for moderately exposed subjects you’ll get a much better contrast between the foreground and background key screen. That will always give you a better key (unless you forget to disable sharpening in your camera–a bad rookie mistake with shooting greenscreens).

Secondly, blue is a more naturally occurring color in clothing and prop design. So if you choose a bluescreen you run the risk of your talent’s jeans becoming invisible. Greenscreen is less likely to cause issues, unless you’re trying to simulate a rainforest with lush green vegetation in every direction.

A third reason: it saves on paint. By that I mean chances are the facility you plan to do your shoot at already has a greenscreen. To repaint blue could blow your budget:)

Is there a time for the blues?

There are certainly situations that call for bluescreens. If your foreground talent is wearing a lot of green, a bluescreen would be a safer choice. If you’re shooting a night scene with a blue themed lighting, it might be a whole lot easier to deal with the blue spill you’ll get from your blue lighting. And in general, blue spill looks much more natural than green spill which tends to make skintone look sickly if not suppressed correctly in post.

A couple of quick tips

Our march coverage of greenscreens and general keying technique will cover off more detail than you ever wanted to know, but here are a couple of quick tips based on what I’ve just covered:

Shoot at the highest resolution color space available. Even if you’re going to shoot the rest of your movie on your uncle’s old mini-DVD camcorder, make the effort to beg, borrow, or rent (no stealing please–this is a civilized filmmaking community) a camera capable of shooting 4:4:4 or RAW for your greenscreen shoot. Once you lose resolution you can never get it back and for visual effects work like keying and tracking, that resolution is critical.

Turn off any kind of sharpening applied in camera. Sharpening (sometimes called softness or some other odd name by different manufacturers) is the great enemy of a good edge in your key. And that–more than anything else besides perspective mismatch–will make your final shot look fake. So turn it off–unless you’re shooting RAW, in which case the sharpening will be applied during debayering so you can worry about that in post.

Look forward to seeing you in March during our It’s Easy Being Green tribute to St Pat.

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